The present invention relates to liquid crystal compounds, liquid crystal media and to liquid crystal displays comprising these media, in particular to displays of the OCB type and to displays of the composite systems type like PDLCs and most particular amongst these to holographic PDLCs.
Liquid Crystal Displays (LCDs) are widely used to display information. Electro-optical modes employed are e.g. the twisted nematic (TN)-, the super twisted nematic (STN)- and the electrically controlled birefringence (ECB)-mode with their various modifications, as well as others. Besides these modes, which all do use an electrical field, which is substantially perpendicular to the substrates, respectively to the liquid crystal layer, there are also electro-optical modes employing an electrical field substantially parallel to the substrates, respectively the liquid crystal layer like e.g. the in-plane switching (IPS)-mode (compare e.g. DE 40 00 451 and EP 0 588 568).
Besides the various different modes using the liquid crystal medium as such, oriented on surfaces, which typically are pre-treated to achieve uniform alignment of the liquid crystal material, there are applications using composite systems of liquid crystal materials of low molecular weight together with polymeric materials such as e.g. polymer dispersed liquid crystal (PDLC)-, nematic curvilinearily aligned phase (NCAP)- and polymer network (PN)-systems, as disclosed for example in WO 91/05 029. These composite systems typically use an electrical field substantially perpendicular to the composite layer. LCDs are used for direct view displays, as well as for projection type displays. Besides these applications LCDs, especially LCDs comprising composite systems like PDLCs and in particular so called holographic PDLC (HPDLC) systems are used in practical applications. HPDLCs are described e.g. in Date, Takeuchi, Tanaka, and Kato, Journal of the SID 7/1 (1999), p. 17 to 22, which is incorporated by reference. These HPDLC displays are generating three bright colors, preferably primary colors, utilizing Bragg reflection. This technique results in excellent bright colors as it does neither need polarizers, nor color filters. A single layer of the periodic structure of polymer and liquid crystal controls the reflection of one particular color. To realize three primary colors consequently three layers, one for each color are required. Each of the three layers has to be addressed independently. This requires three sets of HPDLC films, each with corresponding electrodes. This large number of layers and corresponding electrodes, which is difficult to realize with a good yield in mass production, can beneficiously be reduced when the xe2x80x9ctwo-frequencyxe2x80x9d drive method is applied.
For composite systems a high An of the liquid crystal used is required in order to achieve an efficiently scattering state and to realize a good contrast. Though there have been proposed PDLC-systems with liquid crystal mixtures with low An to improve the so called off axis haze, the predominant problem in most cases is to achieve sufficient contrast in the first place. This is especially the case for PDLC-systems, which are disclosed e.g. in Date, Takeuchi, Tanaka, and Kanto, Journal of the SID 7/1 (1999), p. 17-22. The liquid crystals available typically are characterized by xcex94n values of up to 0.280 or even up to 0.29. This upper limit, however, is still insufficiently low for many applications. Further it has so far only been achieved accepting various compromises with respect to the other properties of the liquid crystal mixtures used. The most typical undesired trade-offs are an insufficiently high clearing point, an unfavourably narrow nematic phase range, a rather high temperature for the lower end of the stability of the nematic phase, too low dielectric anisotropy and hence too high operating voltages, unfavourable elastic constants and last not least too high viscosity values or combinations thereof. Good compatibility with the precursors of the polymers of the composite systems and easy phase separation during the formation of the composite systems are obvious prerequisites for liquid crystals for such applications.
Another promising electro-optical mode used in LCDs is the optically compensated bent (OCB) mode. This mode is described e.g. in Yamaguchi et al., xe2x80x9cWide-Viewing-Angle Display Mode for the Active-Matrix LCD Using Bend-Alignment Liquid-Crystal Cellxe2x80x9d, SID 93, Digest, p. 277 (1993).
This mode is very promising. It is particularly well suited for direct view applications, as it is characterized by a favourable viewing angle dependence. Also the response times are quite short. However for video rate response for the display of changing grey shades the response time still needs to be improved. Compared to a conventional TN display in an OCB displays the amount of deformation of the director is much smaller. Whereas in a TN display the director is oriented almost parallel to the substrates in the unpowered state and changes its direction to almost perpendicular to the substrates upon application of the driving voltage, in an OCB display the director orientation changes to the same final orientation, but starts from an already almost homeotropic bent starting configuraton. Thus, a higher birefringence of the liquid crystal media used is required.
Liquid crystal compounds with a terminal phenyl ring bearing a terminal isothiocyanate group and two F-atoms in ortho position thereto are known from DE 40 27 869.7.
Liquid crystal mixtures consisting predominantly or even entirely of terminally cyano substituted biphenyls and terphenyls are as a rule characterized by suitable high xcex94xcex5 values, but have limited values of xcex94n and already are likely to show insufficient stability at low temperatures, i.e. in most cases either formation of a smectic phase and or crystallisation. Liquid crystal mixtures using large quantities of halogenated tolane compounds with three phenyl rings, which are almost dielectrically neutral, are disclosed, e.g. in the European Patent Application No. EP 99111782.1 are characterized by comparatively low xcex94xcex5 values which are not suitable for most applications and often even show severe problems with respect to the stability of the nematic phase at low temperatures.
Thus, there is a significant need for liquid crystal media with suitable properties for practical applications such as a wide nematic phase range, low viscosities, appropriate optical anisotropy xcex94n according to the display mode used especially a suitably high xcex94n for OCBs and for composite systems like PDLCs and for the latter in particular with suitably large good compatibility with polymer precursors for the composite systems.
Surprisingly, it now has been found that liquid crystal media with high An especially useful for composite systems can be realized which do not exhibit these drawbacks of the materials of the prior art or at least do exhibit them to a significantly lesser degree.
These improved liquid crystal media according to the instant application are realized by using at least two components: a first liquid crystal component (called component A) comprising compounds of formula I, which are strongly dielectrically positive compounds with very high values of xcex94n 
wherein
R1 is n-alkyl, n-alkoxy with 1 to 10 C-atoms, preferably 1 to 7 C-atoms, more preferably 2 to 5 C-atoms, alkenyl, alkenyloxy or alkoxyalkyl with 2 to 7 C-atoms, preferably with 2 to 5 C-atoms or CN, NCS, halogen, preferably F, Cl, halogenated alkyl, halogenated alkenyl or halogenated alkoxy, preferably mono-, di- or oligo-fluorinated alkyl, mono-, di- or oligo-fluorinated alkenyl or mono-, di- or oligo-fluorinated alkoxy, F, Cl, CF3 or OCF3, CF3 OCF2H or OCF3, 
each, independently of each other, are 
Z11 and Z12 each, are independently of each other, a single bond or trans xe2x80x94CHxe2x95x90CHxe2x80x94, in case n1 is 0, Z12 is a single bond
n1 is 0 or 1,
preferably 
and/or preferably 
and simultaneously a second liquid crystal component (called component B), which is a dielectrically positive component comprising, and preferably consisting of terminaly polar substituted bi- or terphenyl compounds, optionally some or all of which are laterally fluorinated, preferably of formula II 
wherein
R2 has the meaning given for R1 under formula I above 
each have the meaning given for 
respectively, above under formula I
and
x2 is CN, F or Cl, preferably CN or Cl, most preferably CN, and
n2 is 0 or 1.
Preferably the liquid crystalline media according to the instant invention contain a compound A comprising, preferably predominantly consisting of and most preferably entirely consisting of compounds of formula I.
In addition to the combination of compounds A and B, the compounds of formula I, wherein at least one of Z11 and Z12 is trans xe2x80x94CHxe2x95x90CHxe2x80x94 and/or wherein at least one of the phenyl rings is substituted by at least one fluorine atom or, which is more preferable, at least two fluorine atoms, or; which is also preferred, wherein at least two of the phenyl rings are bearing at least one fluorine atom each are novel and are one aspect of the present invention.
The compounds of formula I, wherein at least one of Z11 and Z12 is trans xe2x80x94CHxe2x95x90CHxe2x80x94y are prepared from corresponding mesogenic amino compounds. The amino group of these compounds is converted to the isothiocyano group using 1,1-thiocarbonyldiimidazole following the general scheme shown below. Liquid crystal compounds in this application embrace compounds with a liquid crystalline phase by themselves as well as compounds, which are compatible with mesogenic phases, especially with the nematic phase, without decreasing the clearing point unacceptably. The latter compounds have a mesogenic structure and are sometimes called mesogenic compounds. 
wherein
R is alkyl, alkoxy, alkenyl, alkenyloxy or oxaalkyl,
n and m are independently of each other 1 or 2, preferably n+m is 2 or 3,
and
the phenyl rings present may optionally be substituted by up to two F-atoms, preferably the phenyl ring adjacent to the amino group is fluorinated once or twice, preferably in ortho-position to the amino group.
Compounds of formula I wherein Z11 and Z12 both are trans xe2x80x94CHxe2x95x90CHxe2x80x94 are prepared analoguously. These compounds preferably have n1=1.
The amines are prepared using the so called xe2x80x9cHeckxe2x80x9d-reaction as shown in scheme II, 
wherein the parameters are defined as in scheme I above.
Compounds of formula I in which Z11 and Z12 both are a single bond are preferably prepared from the corresponding amino compounds. The amino group of these compounds is converted into the isothiocyanato group by 1,1-thiocarbonyldiimidazole as shown in scheme III. 
Wherein the structural parameters have the same meaning as in formula I above.
The amino compounds are advantageously prepared by cross coupling according to scheme IV. 
Comprising in this application means in the context of compositions that the entity referred to, e.g. the medium or the component, contains the compound or compounds in question, preferably in a total concentration of 10% or more and most preferably of 20% or more.
Predominantly consisting, in this context, means that the entity referred to contains 80% or more, preferably 90% or more and most preferably 95% or more of the compound or compounds in question.
Entirely consisting, in this context, means that the entity referred to contains 98% or more, preferably 99% or more and most preferably 100.0% of the compound or compounds in question.
The compounds of formula I are preferably selected from the group of sub-formulae I-1 to I-3
wherein
R1 has the meaning given under formula I above
Y11 and Y12 are independently of each other H or F
L11 to L16 are independently of each other H or F, preferably one or two of them are F, the others H.
These compounds of formula I-1 preferably are selected from the group of compounds of sub-formulae I-1a to I-1c 
wherein
R1 has the meaning given under formula I above and preferably is n-alkyl with 1 to 5 C-atoms or n-alkoxy with 1 to 4 C-atoms, or 1-E-alkenyl with 2 to 5 C-atoms.
The compounds of formula I-2 are selected preferably from compounds of sub-formulae I-2a I-1i, preferably I-2a to I-2c 
R1 has the meaning given under formula I above and preferably is n-alkyl with 1 to 5 C-atoms or n-alkoxy with 1 to 4 C-atoms, or 1-E-alkenyl with 2 to 5 C-atoms.
The compounds of formula I-3 are preferably selected from compounds of sub-formulae I-3a to I-3d 
wherein
R1 has the meaning given under formula I above and preferably is n-alkyl with 1 to 5 C-atoms.
Most prefered are compounds of formulae I-3a and in particular of formula I-3d.
In a preferred embodiment the liquid crystalline media according to the instant invention contains a component B comprising, preferably predominantly consisting of and most preferably entirely consisting of compounds of formula II.
Preferably these compounds of formula II are chosen from the group of compounds of sub-formulae IIa to IIc. 
wherein
R2 has the meaning given above under formula II and for sub-formula IIa preferably is alkyl or alkoxy and for sub-formulae IIb and IIc preferably alkyl.
In a further preferred embodiment the liquid crystal medium contains a liquid crystal component C which is preferably predominantly consisting of and most preferably entirely consisting of compounds of formula III 
wherein
R31 is alkyl or alkoxy with 1 to 7 C-atoms, alkenyl, alkenyloxy or alkoxyalkyl with 2 to 7 C-atoms,
R32 is Cl, CN or NCS,
R31 is preferably n-alkyl, preferably with 3 to 5 C-atoms, or 1-E-alkenyl with 2 to 5 C-atoms,
R32 is preferably CN or NCS, most preferably NCS, 
is trans-1,4-cyclohexylene or 1,4-phenylene,
Z3 is xe2x80x94CH2CH2xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94 or a single bond, preferably xe2x80x94CH2CH2xe2x80x94 or a single bond and
L31 to L34 are independently of each other H or F, preferably up to two of them are H,
n3 is 0 or 1.
This component C may be dielectrically neutral or dielectrically negative, depending upon the relative amounts of compounds with different meanings of L31 to L34.
Preferably one or two of L31 to L34 are F, preferably L31 is F, the others are H, or L31 and L33 are F or L32 and L34 are F, all other are H.
Component C is used in a concentration of 0 to 30%, preferably 0 to 20% and most preferably from 0 to 10% of the total mixture.
Optionally the inventive liquid crystal medium contains a further component D, which is a dielectrically neutral component and preferably comprises and more preferably consists of dielectrically neutral compounds of formula IV 
wherein
R41 and R42 are, independently of each other, alkyl or alkoxy with 1 to 7 C-atoms or alkenyl, alkenyloxy or alkoxyalkyl with 2 to 7 C-atoms, 
are, independently of each other, trans-1,4-cyclohexylene, 1,4-phenylene, 3-fluoro-1,4-phenylene, 2-fluoro-1,4-phenylene, 2,3-difluoro-1,4-phenylene or 3,5-difluoro-1,4-phenylene, preferably trans-1,4-cyclohexylene, 1,4-phenylene or 3-fluoro-1,4-phenylene,
Z4 is xe2x80x94COOxe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94CHOxe2x80x94 or a single bond, preferably xe2x80x94COOxe2x80x94 or a single bond, trans xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94,
o and p are, independently of each other, 0 or 1.
Component D is used to adjust especially the phase range and the optical anisotropy of the inventive liquid crystal media. Compounds of formula III with o and p both 1 are particularly suited to increase the clearing point of the medium, whereas compounds of formula III with o and p both 0 are particularly suited to decrease the lower limit of the nematic phase range. Especially compounds with Z41xe2x80x94Cxe2x89xa1Cxe2x80x94 are useful to adjust xcex94n of the media.
The concentration of component D in the liquid crystal medium according to the present invention is preferably 0% to 50%, more preferably 0% to 30%, most preferably 0% to 20% and in particular 4% to 16%.
A further optional compound of the liquid crystal media are cyano-substituted trans stilbenes of formula V 
wherein
R5 has the meaning given for R1 in formula I above, preferably R5 is n-alkyl or 1-E-alkenyl 
Independently of each other have the meaning given for 
under formula I above, preferably 
and if present 
n5 is 0 or 1, preferably 0.
Optionally, the inventive media can comprise further liquid crystal compounds in order to adjust the physical properties. Such compounds are known to the expert. Their concentration in the media according to the instant invention is preferably 0% to 30%, more preferably 0% to 20% and most preferably 5% to 15%.
Preferably the liquid crystal medium contains 50% to 100%, more preferably 70% to 100% and most preferably 80% to 100% and in particular 90% to 100% totally of components A and B which contain, preferably predominantly consist of and most preferably entirely consist of one or more of compounds of formulae I and II respectively.
In a preferred embodiment the liquid crystal media according to the instant application comprise at least one compound of each of the following at least five groups.
Group 1 are compounds with two 6-membered rings, preferably 1,4-phenylene rings, which optionally are fluorinated, each bridged by a trans CHxe2x95x90CH group and a terminal NCS group.
Group 2 are compounds with three 6-membered rings, preferably 1,4-phenylene rings, which are optionally fluorinated, which are directly linked to each other and a terminal NCS group.
Group 3 are compounds with two 6-membered rings, which are directly linked to each other or optionally via a CHxe2x95x90CH bridge and which bear a terminal cyano group.
Group 4 are compounds with three 6-membered rings, preferably 1,4-phenylene rings, which are directly linked to each other and having a terminal cyano group.
Group 5 are compounds with two 6-membered rings, which are directly linked and bear a terminal NCS group.
The concentration ranges, in which these groups of compounds are used in the liquid crystal media are as follows:
The liquid crystalline media according to the instant invention are preferably characterized by a clearing point above 80xc2x0 C., more preferably of 90xc2x0 C. or more, especially preferred of 100xc2x0 C. or more, most preferred of 110xc2x0 C. or more and in particular of 120xc2x0 C. or more.
The xcex94n of the liquid crystalline media according to the instant invention is preferably 0.25 or more, more preferably from 0.30 to 0.65, further preferably from 0.32 to 0.60, most preferably from 0.33 to 0.48 and in particular from 0.35 to 0.55.
In a preferred embodiment of the instant onvention the xcex94n of the liquid crystalline media is preferably from 0.30 to 0.60, more preferably from 0.32 to 0.50, most preferably from 0.33 to 0.45 and in particular from 0.35 to 0.40.
The xcex94xcex5, at 1 kHz and 20xc2x0 C., of the liquid crystal medium according to the invention is preferably 6 or more, more preferably 10 or more, most preferably 15 or more and in particular 19 or more.
The liquid crystal media of the state of the art have been limited by low xcex94xcex5 values at high xcex94n values and vice versa. In contrast, the inventive media have pairs of (xcex94n, xcex94xcex5), which are above a line passing through the points (0.290, 18.0) and (0.370, 4.0) in a plot of xcex94xcex5 as a function of xcex94n of the same medium. Preferably they are above a line through (0.290, 20.0) and (0.370, 6.0), most preferably above a line through (0.310, 20.0) and (0.370, 8.0) and in particular above a line through (0.350, 18.0) and (0.390, 8.0).
Preferably the nematic phase of the inventive media extends at least from 0xc2x0 C. to 70xc2x0 C., more preferably at least xe2x88x9220xc2x0 C. to 70xc2x0 C. and most preferably at least from xe2x88x9230xc2x0 C. to 80xc2x0 C., wherein at least means that preferably the lower limit is under cut, wherein the upper limit is surpassed.
In the present application the term dielectrically positive compounds describes compounds with xcex94xcex5 greater than 1.5, dielectrically neutral compounds are compounds with xe2x88x921.5 xe2x89xa6xcex94xcex5xe2x89xa61.5 and dielectrically negative compounds are compounds with xcex94xcex5 less than xe2x88x921.5. The same holds for components. xcex94xcex5 is determined at 1 kHz and 20xc2x0 C. The dielectrical anisotropies of the compounds is determined from the results of a solution of 10% of the individual compounds in a nematic host mixture. The capacities of these test mixtures are determined both in a cell with homeotropic and with homogeneous alignment. The cell gap of both types of cells is approximately 10 xcexcm. The voltage applied is a rectangular wave with a frequency of 1 kHz and a root mean square value typically of 0.5 V to 1.0 V, however, it is always selected to be below the capacitive threshold of the respective test mixture.
For dielectrically positive compounds the mixture ZLI-4792 and for dielectrically neutral, as well as for dielectrically negative compounds, the mixture ZLI-3086, both of Merck KGaA, Germany can be used as host mixtures, respectively. The dielectric permittivities of the compounds are determined from the change of the respective values of the host mixture upon addition of the compounds of interest and are extrapolated to a concentration of the compounds of interest of 100%.
Components having a nematic phase at the measurement temperature of 20xc2x0 C. are measured as such, all others are treated like compounds.
The term threshold voltage refers in the instant application to the optical threshold and is given for 10% relative contrast (V10) and the term saturation voltage refers to the optical saturation and is given for 90% relative contrast (V90) both, if not explicitly stated otherwise. The capacitive threshold voltage (V0, also called Freedericksz-threshold VFr) is only used if explicitly mentioned.
The ranges of parameters given in this application are all including the limiting values, unless explicitly stated otherwise.
Throughout this application, unless explicitly stated otherwise, all concentrations are given in mass percent and relate to the respective complete mixture, all temperatures are given in degrees centigrade (Celsius) and all differences of temperatures in degrees centigrade. All physical properties have been and are determined according to xe2x80x9cMerck Liquid Crystals, Physical Properties of Liquid Crystalsxe2x80x9d, Status Nov. 1997, Merck KGaA, Germany and are given for a temperature of 20xc2x0 C., unless explicitly stated otherwise. The optical anisotropy (xcex94n) is determined at a wavelength of 589.3 nm. The dielectric anisotropy (xcex94xcex5) is determined at a frequency of 1 kHz. The threshold voltages, as well as all other electro-optical properties have been determined with test cells prepared at Merck KGaA, Germany. The test cells for the determination of xcex94xcex5 had a cell gap of 22 xcexcm. The electrode was a circular ITO electrode with an area of 1.13 cm2 and a guard ring. The orientation layers were lecithin for homeotropic orientation (xcex5∥) and polyimide AL-1054 from Japan Synthetic Rubber for homogeneuous orientation (xcex5xe2x8axa5). The capacities were determined with a frequency response analyser Solatron 1260 using a sine wave with a voltage of 0.3 Vrms. The light used in the electro-optical measurements was white light. The set up used was a commercially available equipment of Otsuka, Japan. The characteristic voltages have been determined under perpendicular observation. The threshold (V10)xe2x80x94mid grey (V50)xe2x80x94 and saturation (g90) voltages have been determined for 10%, 50% and 90% relative contrast, respectively.
The liquid crystal media according to the present invention can contain further additives and chiral dopants in usual concentrations. The total concentration of these further constituents is preferably from 0% to 10%, more preferably 0.1% to 6%, based on the total mixture. The concentrations of the individual compounds used each are preferably from 0.1 to 3%. The concentration of these and of similar additives is not taken into consideration for the values and ranges of the concentrations of the liquid crystal components and compounds of the liquid crystal media in this application.
The inventive liquid crystal media according to the present invention may consist of several compounds, preferably of 3 to 30, more preferably of 5 to 20 and most preferably of 6 to 14 compounds. These compounds are mixed in conventional way. Generally, the required amount of the compound used in the smaller amount is dissolved in the compound used in the greater amount. In case the temperature is above the clearing point of the compound used in the higher concentration, it is particularly easy to observe completion of the process of dissolution. It is, however, also. possible to prepare the media by other conventional ways, e.g. using so called pre-mixtures, which can be e.g. homologous or eutectic mixtures of compounds or using so called multi-bottle-systems, the constituents of which are ready to use mixtures themselves.
By addition of suitable additives, the liquid crystal media according to the instant invention can be modified in such a way, that they are usable in all known types of liquid crystal displays, either using the liquid crystal media as such, like TN-, TN-AMD, ECB-, VAN-AMD and in particular in composite systems, like PDLC-, NCAP- and PN-LCDs and especially in HPDLCs.
The melting point T(C,N), the transition from the smectic (S) to the nematic (N) phase T(S,N) and the clearing point T (N,l) of the liquid crystals are given in degrees centigrade.
The entire disclosure of all applications, patents and publications, cited above and below, and of corresponding European Application No. EP 00102952.9, filed Feb. 14, 2000; European Application No. EP 00109164.4, filed May 8, 2000; and European Application No. EP 1026408.4, filed Dec. 5, 2000, are hereby incorporated by reference.
In the present application and especially in the following examples, the structures of the liquid crystal compounds are represented by abbreviations also called acronyms. The transformation of the abbreviations into the corresponding structures is straight forward according to the following two tables A and B. All groups CnH2n+1 and CmH2m+1 are straight chain alkyl groups with n respectively m C-atoms. The interpretation of table B is self evident. Table A does only list the abbreviations for the cores of the structures. The individual compounds are denoted by the abbreviation of the core followed by a hyphen and a code specifying the substituents R1, R2, L2 and L2 follows:
The liquid crystal media according to the instant invention do contain preferably
four or more compounds selected from the group of compounds of tables A and B and/or
five or more compounds selected from the group of compounds of table B and/or
two or more compounds selected from the group of compounds of table A.